The need to measure both the torque and the angular position of a steering shaft is important to automotive applications utilizing an electric power steering system (EPS). A number of standalone sensors have been developed related to either determining torque or calculating the angular position of a steering wheel. Generally, it is necessary to calculate the torque in order to determine the amount of electrical assist to apply when a driver turns a steering wheel. In determining torque, the driver typically turns the steering wheel which is connected to an input shaft. The input shaft is coupled to an output shaft which is connected to a steering mechanism. The input and output shafts are generally coupled together with a torsion bar and the torsion bar allows for relative rotation between the shafts. The input shaft may rotate with respect to the output shaft by a predetermined number of degrees, e.g. +/−12 degrees. An example of a torque sensor is disclosed in U.S. Patent Application Publication No. 2004/0250631 filed on Jul. 13, 2004 which is assigned to the assignee of the present invention and is hereby incorporated as a reference. An example of a position sensor is disclosed in U.S. Pat. No. 6,720,763 which is assigned to the assignee of the present invention and is hereby incorporated as a reference.
Position sensors are used for determining the angular position of the shaft as a user turns a steering wheel. Examples of position sensors are disclosed in U.S. Pat. No. 5,930,905 (the '905 patent) to Zabler et al., and in U.S. Pat. No. 6,630,823 (the '823 patent) to Tateishi et al. The '905 patent discloses a gear having a plurality of teeth coupled to a shaft. The teeth engage a plurality of additional teeth disposed on a second gear and a third gear. A pair of absolute sensors are positioned in proximity to the second gear and the third gear for generating an output that corresponds to the angular positions of the gears. As long as the number of teeth on each of the gears are known, it is possible to calculate the angular position of the shaft based on the outputs of the absolute sensors.
The '823 patent discloses a gear having a plurality of teeth coupled to a shaft. As the shaft rotates, the teeth on the gear engages with additional gear teeth disposed on a second gear. A bevel gear disposed on a different plane from the second gear engages a change gear. A first sensing element is positioned in proximity with the change gear for outputting a first detection signal which repeats continuously. A screw is positioned through the change gear and rotates with the change gear as the change gear rotates. A driven body is coupled with the screw and is axially displaced in response to rotating the change gear. A second sensing element is positioned in proximity to the driven body and outputs a second detection signal which gradually increases or decreases. A detection circuit determines the angular position of the shaft based on the first and the second detection circuits.
Although these prior art position sensors are useful, an opportunity exists for a position sensor that minimizes the number of gear teeth disposed on a gear while providing the angle position of the shaft with accuracy. By reducing the number of gear teeth on a gear, the sensor is not susceptible to losing accuracy as a result of the teeth wearing down over time. Additionally, an opportunity exists for combining the torque and position sensing capabilities into an integrated sensor to occupy less package space in a vehicle. Finally, by combining the functionality of a sensor which calculates both the torque and angular position of a shaft, common parts may be utilized within the sensor for providing additional cost savings.